I have reported the issue to Adaptable and they have solved it very quickly.
I am using adaptable which I downloaded less than a week ago while upgrading to 3.5 and I am now experiencing this issue. Atto drop-downs work in clean but not adaptable.
After inspecting each of the files mentioned in https://bitbucket.org/covuni/moodle-theme_adaptable/issues/909/atto-drop-down-menu it looks as though my files have all the latest changes however the Atto dropdowns still do not work.
The language submenu was already disabled, so that isn't the issue.
Just upgraded to Moodle 3.5 and have the same challenge.
How/where can I get the Atto update? my version is 2018051401 and I can not find a way to update it